Cretaceous epithermal Au-Ag mineralization in the Muju-Yeongam district, Republic of Korea: Muju mineralized area.

Abstract

The gold-silver vein deposits of the Muju area, about 180km SSE of Seoul, are examples of the epithermal gold-silver deposits of the Republic of Korea. The deposits are composed of quartz and calcite veins that crosscut Precambrian gneiss and Cretaceous volcanic rocks (dacite porphyry of ≈ 130 Ma, and rhyodacitic crystal tuff) and intrusive rocks (porphyritic granite of ≈ 102 Ma). Mineralization occurred in four stages (I, II, III, and IV) which filled preexisting fault fracture zones. Hydrothermal wall-rock alteration up to 10m wide is characterized by sericitic, silicic, chloritic and pyritic assemblages. Pipe-like vuggy silica alteration characteristically occurs at the innermost and surface zones of dacite porphyry-hosted quartz veins. K-Ar dating of alteration sericite (≈ 103 Ma) indicates a late Cretaceous age for ore mineralization, which is probably associated with the intrusion of the porphyritic granite. Fluid inclusion and stable isotope data show that electrum, argentite, sphalerite and galena were deposited at temperatures between 260° and 180°, from fluids with salinities between 3.2 and 0.5 wt. % NaCl equiv.. Fluid inclusion evidence of boiling suggests pressures of ≈ 40 bars during late portions of stage II mineralization, corresponding to depths between 150 and 400m, assuming lithostatic and hydrostatic loads. Equilibrium thermodynamic interpretation of mineral assemblages indicates that the gold-silver-depositing fluids had a pH of 4.4 to 5.4 and log activities of S2 and 02 of -12 to -16 atm and -37 to -38.7 atm, respectively. Gold-silver deposition was a combined result of the cooling of ore fluids through mixing with less-evolved meteoric waters and the decrease of sulfur activity due to sulfide precipitation. Sulfur isotope compositons of stage II sulfide minerals range from 0.5 to 5.9 per mil, yielding a δ34Σs value near 4 per mil which indicates a local igneous source for sulfur. The oxygen and hydrogen isotope values of hydrothermal fluids (δ18O=-6.6 to -1.3 per mil, δD=-83 to -73 per mil) are consistent with meteoric water dominance approaching unexchanged meteoric water values.